Mechanistic study of novel small-molecule compounds in potentiating antibiotic activity to overcome resistance in Gram-positive bacteria

Abstract

The discovery of antibiotics has been considered as one of the most successful medicine advancements in 20th century. However, people are still troubled with bacterial infection as a result of emerging antibiotic resistance, intracellular bacterial survival as well as biofilm formation. While the conventional approach of looking for novel antimicrobials to replace the old ones may not be an efficient solution, we proposed antibiotic-adjuvant approach in combating antimicrobial resistance in this study. SA-558, a novel small-molecule compound, which works in combination with gentamicin against multidrug resistant S. aureus was identified from compound screening. It is characterized that SA-558 potentiates antibacterial activity of two important classes of antibiotics, that are aminoglycosides and polymyxins. SA-558 activity can be extended to Gram-positive bacteria, including Streptococcus, Bacillus and Listeria and absent in Gram-negative bacteria due to the exclusion by outer membrane. SA-558 is also active against intracellular bacteria and biofilm formation. SA-558 molecular mechanism has been unveiled that ionization and entry of SA-558 in bacterial cells lead to an increase in intracellular proton concentration and dissipate the proton gradient, resulting in a counter-increase in membrane potential. With hyperpolarized membrane, an increase in aminoglycosides uptake is resulted. It is shown that SA-558 activity occurs in pH-dependent manner, with enhanced activity at lower pH and this explained for its activity against intracellular bacteria and biofilm formation. For polymyxins-potentiating activity, it is shown that as a result of hyperpolarization, SA-558 rendered the cell surface charge of bacterial cells more negative and enhanced polymyxins electrostatic interaction. The treatment of SA-558 alone and in combination with gentamicin have been shown to be non-toxic in mice. In the murine infection model of multidrug resistant S. aureus, it is demonstrated that combination of SA-558 and gentamicin can significantly improve survival in mice from lethal infection and reduce bacterial load in important organs, that are liver, spleen and kidney, against nonlethal infections. It is also found that SA-558 alone is efficacious against lethal infection of multidrug resistant S. aureus and bacterial clearance was observed in some mice against non-lethal infection. The serendipitous SA-558 alone activity was attributed to its enhanced activity at lower pH, which is often observed in the bacterial infection sites. It is envisioned that SA-558 is a potential drug to be used alone or in combination with gentamicin for antimicrobial therapy, bringing new possibility in the ‘post-antibiotic’ era.